This invention relates generally to apparatus which mixes two liquids, and specifically to such an apparatus which mixes and activates a liquid polymer with water to provide a diluted activated polymer of particular use in the treatment of water for various purposes.
Solutions in water of certain high molecular weight water soluble resins are of considerable technical use as agents for viscosification, solids-liquids separation processes, additives for drilling fluids, for assisting oil recovery from subterranean formations, etc.
It is becoming increasingly recognised that the efficiency in the applications of the polymeric molecules of which these resins are comprised, and therefore their economy of use, is a function of the conformation of the molecules in solution at the time of the application of these solutions to their respective substrates. Conformation includes such features as the dimensions of the molecules in their solvated state, their degree of entanglement, the extent of chain branching, their ionicity and the relative dispositions of polyion and counter ion. Such features are described in the papers by Muller in Polymer Bulletin 1984, pages 391-5, by McCormick in Polymer Engineering Materials Science 1984, pages 471-474, and by Casky and Primus in Environmental Progress, vol. 2 1986, pages 98-103, for example.
The above mentioned resins are sold as concentrates in various physical forms such as finely ground powders, beads, concentrated solutions gels, water in oil emulsions of gels, oil dispersions of powders etc. They may be synthetic or natural materials. The synthetic materials are typified by the water soluble homopolymers of cationic acrylates or methacrylates, salts of acrylic acid, salts of 2 acrylamido 2 methyl propane sulphonic acid, acrylamide etc. or copolymers involving such monomeric species, all of which at high molecular weight (i.e. above 100,000) have appreciable flocculating or viscosifying action which is a function of their conformation and bonding capability. Natural materials would be typified by water soluble polysaccharides and graft copolymers thereof with synthetic moieties, certain natural gums, polysaccharides of microbial origin, etc.
Many applications require the polymer to be applied to the substrate as a dilute aqueous solution, commonly 2% or less and more typically 0.5% or less. To this end, various devices have been proposed for effecting solution and dilution to the required level of concentration. In one such apparatus, disclosed in U.S. Pat. No. 4,113,688, solutions are prepared from a gel form of a synthetic polyelectrolyte. The machinery employed is critical to the final polymer properties and a two stage gel cutting and dissolving machine is described therefor. Various other polymer powder dissolving machines are sold which feature devices for intimate contacting of the powder with water to avoid clumping of polymer gel so as to avoid the necessity for extreme conditions of agitation which would otherwise damage the polymer molecules. Damage to polymer molecules by high shear rate conditions in solution is well known as exemplified in the paper by Nakano and Minoura in the Journal of Applied Polymer Science, 1978, vol. 22, pp. 2207-2215.
In U.S. Pat. No. 3,852,234, there is described an apparatus and procedure for dissolution by inversion of water in oil emulsions of polymers and gums wherein a water-surfactant mix of high turbulence as characterized by a Reynolds number of greater than 50,000, is contacted by the emulsion prior to passage through a static mixer. Maximization of "activation" of the polymer is said to occur not within the mixer but some time after the egress of the mix from the apparatus, presumably in a subsequent storage vessel. Here activation is defined in terms of maximization of viscosity after exit and also in terms of settling tests where the solution is to be used as a sedimentation aid.
In U.S. Pat. No. 4,522,502, there is described an apparatus for mixing and feeding polymer dispersions and water, for the purpose of water treatments wherein high energy but low shear rate conditions are employed. This invention features a chamber containing a rotor which is designed to produce good polymer activation within a single mixing zone with minimum damage to molecular structure.
It is well understood by those acquainted with the art of polymer activation that commercially available solutions of polymers or those concentrated solutions of polymers resulting from commercially available powder dissolution systems or those polymer dispersions which are inverse emulsions, vary considerably in the ease with which they can be diluted or, inverted and then diluted, as the case may be. Such factors rest on the viscosities of the above mentioned dispersions, on their coherence, their viscoelasticity and their osmotic tendencies.
It is further well understood that because of these properties, the dissolution and inverting actions cannot be undertaken without application of hydrodynamic shearing action. For this reason problems arise due to the balance between forces necessary to cause the dispersion of the concentrates and those which are regarded as excessive in relation to the fragility of the molecules and aggregates thereof, the size and conformation of which are directly related to optimum performance.
The problem is complex in that during the course of dissolution and activation of the polymers, change in the conformation of the polymer molecule takes place very rapidly. These changes in the conformation are outwardly manifested in a rapid rise in the viscosity and viscoelasticity of the solution.
As activation proceeds toward optimum, the polymer molecule becomes more extended and is subject to mechanical damage. If hydrodynamic conditions are maintained at a level (shear rate) that is adequate to assure initial dispersion and activation for too long a period after the molecular conformation begins to change then polymer degradation will almost certainly result.